1. Field of the Invention
The present invention relates to an image forming method and apparatus for forming an image with an electrophotographic method on a medium (recording paper) to be recorded by using toner. More specifically, the present invention relates to an image forming method and apparatus for forming an image in accordance with image data corresponding to black and other plurality of colors and a predetermined resolution characteristic.
2. Description of Related Art
As a conventional image forming apparatus of this kind, there is given examples such as an image forming apparatus comprising: a latent image forming unit for forming an electrostatic latent image corresponding to each color on an image carrier, in accordance with image data corresponding to black and other plurality of colors (such as yellow, magenta, and cyan) and a predetermined resolution characteristic; a developing unit for developing the electrostatic latent image formed on the image carrier by the latent image forming unit by adsorbing toner of each color thereon; and a transfer unit for transferring the toner adsorbed to the image carrier by the developing unit, to a medium (recording paper) to be recorded.
According to the conventional image forming apparatus thus constituted, in accordance with the image data corresponding to the aforementioned each color and the resolution characteristic (such as a dither matrix), the latent image forming unit forms the electrostatic latent image on the image carrier, and the developing unit performs development by adsorbing the toner of the aforementioned each color on the electrostatic latent image. In this way, by transferring the toner adsorbed on the image carrier to the medium (recording paper) to be recorded by the transfer unit, toners of the aforementioned each color are arranged on the medium to be recorded in a predetermined positional relation, and a color image is formed on the medium to be recorded.
In addition, the conventional image forming apparatus of this kind proposes that beam diameters for forming the electrostatic latent image is differentiated between a case of forming black color (referred to as three-color black hereafter) by transferring three colors of yellow, magenta, and cyan to adjacent positions on the medium to be recorded, and a case of forming black color (referred to as monochromatic black hereafter) by transferring only toner of black to the medium to be recorded (for example see Japanese Patent Application laid-Open No. 2000-62250).
Incidentally, a combination of the aforementioned three-color black and the monochromatic black can be considered. Specifically, there is a conceivable case such that after forming three-color black by transferring yellow, magenta, and cyan to the adjacent positions, the toner of black is further transferred thereon to obtain high density black color (referred to as four-color black hereafter). However, a problem is involved in this case as follows. Namely, since the toner of each color are generally charged with the same polarity, when the toner of black is transferred on the three-color black, electrostatic repulsion of the toners of yellow, magenta, and cyan is exerted on the toner of black. Therefore, a density of black is deteriorated in a portion where the toner of black is flipped by the aforementioned repulsion.